I would recommend using software mixing (not FSOUND_HW2D or FSOUND_HW3D), then playing the sounds/streams paused, then unpausing them from a DSP callback. That would allow you to get exact synchronization.
In fmod ex it is a bit simpler, you just have to put System::lockDSP and System::unlockDSP around your playsound calls.
The best way to do that is to use FMOD Ex and see realtimestitching example. FMOD 3 doesnt have the gapless stitching flexibility that FMOD Ex does.
Also for synchronized starts that is easier with simply calling System::lockDSP / unlockDSP.
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